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      1 //===-- ConstantHoisting.h - Prepare code for expensive constants ---------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This pass identifies expensive constants to hoist and coalesces them to
     11 // better prepare it for SelectionDAG-based code generation. This works around
     12 // the limitations of the basic-block-at-a-time approach.
     13 //
     14 // First it scans all instructions for integer constants and calculates its
     15 // cost. If the constant can be folded into the instruction (the cost is
     16 // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
     17 // consider it expensive and leave it alone. This is the default behavior and
     18 // the default implementation of getIntImmCost will always return TCC_Free.
     19 //
     20 // If the cost is more than TCC_BASIC, then the integer constant can't be folded
     21 // into the instruction and it might be beneficial to hoist the constant.
     22 // Similar constants are coalesced to reduce register pressure and
     23 // materialization code.
     24 //
     25 // When a constant is hoisted, it is also hidden behind a bitcast to force it to
     26 // be live-out of the basic block. Otherwise the constant would be just
     27 // duplicated and each basic block would have its own copy in the SelectionDAG.
     28 // The SelectionDAG recognizes such constants as opaque and doesn't perform
     29 // certain transformations on them, which would create a new expensive constant.
     30 //
     31 // This optimization is only applied to integer constants in instructions and
     32 // simple (this means not nested) constant cast expressions. For example:
     33 // %0 = load i64* inttoptr (i64 big_constant to i64*)
     34 //===----------------------------------------------------------------------===//
     35 
     36 #ifndef LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
     37 #define LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
     38 
     39 #include "llvm/Analysis/BlockFrequencyInfo.h"
     40 #include "llvm/Analysis/TargetTransformInfo.h"
     41 #include "llvm/IR/Dominators.h"
     42 #include "llvm/IR/PassManager.h"
     43 
     44 namespace llvm {
     45 
     46 /// A private "module" namespace for types and utilities used by
     47 /// ConstantHoisting. These are implementation details and should not be used by
     48 /// clients.
     49 namespace consthoist {
     50 /// \brief Keeps track of the user of a constant and the operand index where the
     51 /// constant is used.
     52 struct ConstantUser {
     53   Instruction *Inst;
     54   unsigned OpndIdx;
     55 
     56   ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { }
     57 };
     58 
     59 typedef SmallVector<ConstantUser, 8> ConstantUseListType;
     60 
     61 /// \brief Keeps track of a constant candidate and its uses.
     62 struct ConstantCandidate {
     63   ConstantUseListType Uses;
     64   ConstantInt *ConstInt;
     65   unsigned CumulativeCost;
     66 
     67   ConstantCandidate(ConstantInt *ConstInt)
     68     : ConstInt(ConstInt), CumulativeCost(0) { }
     69 
     70   /// \brief Add the user to the use list and update the cost.
     71   void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
     72     CumulativeCost += Cost;
     73     Uses.push_back(ConstantUser(Inst, Idx));
     74   }
     75 };
     76 
     77 /// \brief This represents a constant that has been rebased with respect to a
     78 /// base constant. The difference to the base constant is recorded in Offset.
     79 struct RebasedConstantInfo {
     80   ConstantUseListType Uses;
     81   Constant *Offset;
     82 
     83   RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
     84     : Uses(std::move(Uses)), Offset(Offset) { }
     85 };
     86 
     87 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
     88 
     89 /// \brief A base constant and all its rebased constants.
     90 struct ConstantInfo {
     91   ConstantInt *BaseConstant;
     92   RebasedConstantListType RebasedConstants;
     93 };
     94 }
     95 
     96 class ConstantHoistingPass : public PassInfoMixin<ConstantHoistingPass> {
     97 public:
     98   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
     99 
    100   // Glue for old PM.
    101   bool runImpl(Function &F, TargetTransformInfo &TTI, DominatorTree &DT,
    102                BlockFrequencyInfo *BFI, BasicBlock &Entry);
    103 
    104   void releaseMemory() {
    105     ConstantVec.clear();
    106     ClonedCastMap.clear();
    107     ConstCandVec.clear();
    108   }
    109 
    110 private:
    111   typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType;
    112   typedef std::vector<consthoist::ConstantCandidate> ConstCandVecType;
    113 
    114   const TargetTransformInfo *TTI;
    115   DominatorTree *DT;
    116   BlockFrequencyInfo *BFI;
    117   BasicBlock *Entry;
    118 
    119   /// Keeps track of constant candidates found in the function.
    120   ConstCandVecType ConstCandVec;
    121 
    122   /// Keep track of cast instructions we already cloned.
    123   SmallDenseMap<Instruction *, Instruction *> ClonedCastMap;
    124 
    125   /// These are the final constants we decided to hoist.
    126   SmallVector<consthoist::ConstantInfo, 8> ConstantVec;
    127 
    128   Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
    129   SmallPtrSet<Instruction *, 8>
    130   findConstantInsertionPoint(const consthoist::ConstantInfo &ConstInfo) const;
    131   void collectConstantCandidates(ConstCandMapType &ConstCandMap,
    132                                  Instruction *Inst, unsigned Idx,
    133                                  ConstantInt *ConstInt);
    134   void collectConstantCandidates(ConstCandMapType &ConstCandMap,
    135                                  Instruction *Inst);
    136   void collectConstantCandidates(Function &Fn);
    137   void findAndMakeBaseConstant(ConstCandVecType::iterator S,
    138                                ConstCandVecType::iterator E);
    139   unsigned maximizeConstantsInRange(ConstCandVecType::iterator S,
    140                                     ConstCandVecType::iterator E,
    141                                     ConstCandVecType::iterator &MaxCostItr);
    142   void findBaseConstants();
    143   void emitBaseConstants(Instruction *Base, Constant *Offset,
    144                          const consthoist::ConstantUser &ConstUser);
    145   bool emitBaseConstants();
    146   void deleteDeadCastInst() const;
    147   bool optimizeConstants(Function &Fn);
    148 };
    149 }
    150 
    151 #endif // LLVM_TRANSFORMS_SCALAR_CONSTANTHOISTING_H
    152